Residential ground fault circuit interrupter (GFCI) circuit breakers presently rely on a manual “Press to Test” pushbutton actuation by a homeowner to verify proper operation of the ground fault detection circuit. During the life of this circuit breaker several electrical stress sources (for example, lightning, line disturbances, etc.) can render this circuit inoperable. Unfortunately, the homeowner may not routinely exercise the aforementioned “Press To Test” pushbutton during this inoperative period, and therefore may not recognize the circuit failure.
Present residential GFCI circuit breakers do not provide an internal watchdog function that detects the failure(s) of the major circuit components, and non-functioning GFCI breakers have been observed in the field. The “Press To Test” manual pushbutton is not regarded as a fail-safe functional test, since many homeowners do not routinely exercise this manual test.
Typical residential GFCI circuit breakers can be rendered inoperable due to failed circuits because of environmental surges, defective components, and manufacturing issues. These GFCI circuit breakers will continue to provide branch power, but with no ground fault interruption capability.
FIG. 1 shows a conventional Ground Fault Circuit Interrupter. In FIG. 1, element 100 represents a ground fault detector, which may be an integrated circuit, and specifically may be a Fairchild RV 4141A Low Power Ground Fault Interrupter. Ground fault detector 100 is electrically coupled to transformer 120, which is arranged to measure whether a ground fault arises on a circuit. The circuit may be any household item, for instance, a hair dryer, or may be any other appropriate electrical device. Line 170 represents a load hot line, and line 180 represents a load neutral line. Lines 170 and 180 both pass through transformer 120. Line 180 is continuous with line 185 which is a line neutral line, and line 170 is continuous with line 175, which is a switched hot line. When a ground fault situation exists on the load, a current difference between lines 170 and 180 exceeds a minimum threshold. This current difference between lines 170 and line 180 induces transformer 120 to output a signal that causes ground fault detector 100 to emit a breaking signal. The breaking signal causes solenoid 130 to activate plunger 140 that causes a breaker to break the circuit. Alternatively, solenoid 130 may operate to pull an armature directly or indirectly through another trigger device. The breaking signal from ground fault detector 100 turns on silicon control rectifier 150 (also referred to herein as SCR 150).
In FIG. 1, a power supply is formed by solenoid 130, bridge 135, and other discrete elements inside detector 100. Ground fault detector 100 effectively amplifies the electrical current difference in hot and neutral wires passing through transformer 120 (also referred to herein as a differential toroid) and compares the electrical current difference to a predetermined threshold. When an excessive difference is detected (for example, 5 ma), ground fault detector 100 outputs a positive going signal that is applied to the gate of SCR 150. This will cause SCR 150 to fire, forcing an actuation current to flow through solenoid 130. In turn, plunger 140 will strike an appropriate mechanical lever causing the breaker to trip.
A conventional ground fault detector circuit may also include a press-to-test button 110 (also referred to herein as a push-to-test button, or PTT button), which serves as a manual self-test feature. Press-to-test button 110 in FIG. 1 operates to test ground fault detector 100, by causing a current imbalance through transformer 120 which causes a ground fault difference current signal to be sent to ground fault detector 100. Thus, the PTT button actuation causes an actual imbalance of current by passing additional current through transformer 120 using discrete elements 111, 112, 113, and 114 arranged in series and wire 115, without returning the current back through transformer 120. Ground fault detector 100 senses this imbalance and fires SCR 150, thus causing a breaker trip condition. In this situation, the solenoid 130 would activate plunger 140 which causes a break in the circuit. The user who has pressed test button 110 to test the ground fault detector circuit is aware of the circuit breaking by seeing the breaker break, and is therefore able to confirm operation of ground fault detector 100. The user is also able to reset the circuit by resetting the breaker manually so that continued operation of the electrical appliance at the load is possible. Although the breaker installation instructions form asks the homeowner to periodically exercise PTT button 110, it is seldom actually done. Thus, failures in the GFCI circuit can render the circuit inoperable without any notice to the homeowner.
FIG. 1 also includes second transformer 160 which is adapted to determine whether a fault condition exists which does not create a sufficient current difference in transformer 120 to meet the threshold for causing ground fault detector 100 to detect a fault. The coupled signal from second transformer 160 to transformer 120 provides a positive feedback loop around an operational amplifier (also referred to herein as a sense amplifier) in the RV4141 embodiment of ground fault detector 100. Therefore, when a grounded neutral short (also referred to herein as a ground-to-neutral fault) exists, the sense amplifier (used to process imbalance current from transformer 120) turns into an oscillator circuit. A ground fault current is detected by transformer 120 and amplified by the sense amplifier. Second transformer 160 and transformer 120 become mutually coupled, producing a positive feedback loop around the sense amplifier. The newly created feedback loop causes the sense amplifier to oscillate at a frequency determined by a secondary inductance of second transformer 160 and capacitor 161. This oscillation frequency may be about 8 KHz.
The circuit of FIG. 1 also includes various diodes, resistors and capacitors arranged in a manner to enable the circuit to function appropriately. The values of the resistors and capacitors, and the arrangements of all of the elements is conventional, and the precise arrangement may be determined by one of ordinary skill in the art without undue experimentation.